(1) Field of the Invention
The present invention relates to a valve mechanism for an internal combustion engine, which valve mechanism is capable of opening and closing the intake valves and the exhaust valves of the internal combustion engine with different timings in accordance with the operation state of the engine.
(2) Description of the Related Art
Recently, a valve mechanism equipped with a variable valve lift and timing mechanism, in which the operational characteristics (open/close timing and open duration) of the intake valves and the exhaust valves of an internal combustion engine (hereinafter simply called “engine”) can be selected according to the load state of the engine and the velocity state, has been developed and is in practical use.
In such a valve mechanism, as one of such mechanisms for selecting operation characteristics, a mechanism with two types of cams, one of which is a low-velocity cam whose cam profile is suitable for low-velocity operation of the engine and the other of which is a high-velocity cam whose cam profile is suitable for high-velocity operation of an engine, has been developed (e.g., see Japanese Patent Application Laid-open No. 2003-343226). Depending upon rotation state of the engine, the cams are selectively used to open and close the engine valves.
Now, referring to FIG. 3 through FIG. 5, a description will be made of a previous valve mechanism. As shown in FIG. 3 and FIG. 4, two intake valves 11 and 12 and two exhaust valves 21 and 22 are provided for the cylinder head 10 placed upward of each cylinder of the engine. To drive these intake valves 11 and 12 and exhaust valves 21 and 22, a valve mechanism 30 is prepared.
The valve mechanism 30 has an intake valve driving system for driving the intake valves 11 and 12 and an exhaust valve driving system for driving the exhaust valves 21 and 22. The intake valve driving system includes: a cam shaft 31; cams 31a through 31c fixed to the cam shaft 31; a rocker shaft 32; rocker arms 33 through 35 which are supported by the rocker shaft 32 in such a manner that the rocker arms 33 through 35 make a see-saw-like movement, following the rotation of the cams 31a through 31c. On the other hand, the exhaust valve driving system includes: a cam shaft 31 which is commonly used between the intake valve driving system and the exhaust valve driving system; cams 31d and 31e fixed to the cam shaft 31; rocker arms 37 and 38 which are supported by the rocker shaft 36 in such a manner that the rocker arms 37 and 38 (not illustrated in FIG. 4) make a see-saw-like movement, following the rotation of the cams 31d and 31e. 
Further, at the intake valve driving system of the valve mechanism 30, a variable valve lift and timing mechanism 40 including a connection switching mechanism 41 is provided. A description will made hereinbelow of the variable valve mechanism 40.
Of the rocker arms 33 through 35 which drive the intake valves, at one end of each rocker arm 33 and 34, adjustment screws 33a and 34a are provided, respectively, through which adjustment screws 33a and 34a the ends of the stems of the intake valves 11 and 12 are made to come into contact with one end of each rocker arm 33 and 34, respectively. As a result, the intake valve 11 opens and closes in accordance with the see-saw-like movement of the rocker arm 33, and the intake valve 12 opens and closes in accordance with the see-saw-like movement of the rocker arm 34.
Further, at the other ends of the rocker arms 33 and 34, the rollers 33b and 34b are provided, respectively. The rollers 33b and 34b come into contact with the low-velocity cam 31a and 31b, respectively, each of which has a low-velocity cam profile for a low-velocity operation of the engine. When the rocker arms 33 and 34 make a see-saw-like movement in response to the rotation of the low-velocity cam 31a and 31b, the intake valves 11 and 12 open with characteristics suitable for low-velocity operation.
On the other hand, the rocker arm (second rocker arm) 35 has an engaging protrusion 35a which is capable of engaging with the rocker arms 33 and 34. The roller 35b provided at the other end of the rocker arm 35 comes into contact with a high-velocity cam 31c which has a high-velocity cam profile for a high-velocity operation of the engine.
In addition, as shown in FIG. 5(a) and FIG. 5(b), a cylinder 50 with an opening 53 is provided for the rocker arms 33 and 34 at a position at which one end of the rocker arm 35 comes into contact with the cylinder 50, and a piston 51 is fitted in the cylinder 50.
The cylinder 50 is constructed so that hydraulic oil (here, lubricating oil is commonly used) is supplied thereto from the rocker shaft 32 via an oil passage (communicating path) 32b. When hydraulic oil is supplied into the cylinder 50, the piston 51 is lifted by the supplied oil, as shown in FIG. 5(b), thereby closing the opening 53. Further, when the oil pressure in the cylinder 50 is released to the air, the piston 51 is pushed down by a force applied from the return spring 52, thereby opening the opening 53.
The piston 51 in the cylinder 50 and an oil pressure adjusting device (not illustrated) for adjusting oil pressure in the cylinder 50 form a connection switching mechanism 41 which selects the connection state between the rocker arms 33 and 34 and the rocker arm 35. The connection switching mechanism 41 and the intake valve driving system form a variable valve mechanism 40.
With the above-described arrangement, when the oil pressure in the cylinder 50 is exhausted by the oil pressure adjusting device, a space is formed at the opening 53 of the cylinder 50 [see FIG. 5(a)]. In this case, when the rocker arm 35 makes a see-saw-like movement, following the rotation of the high-velocity cam 31c, the engaging protrusion 35a enters the thus formed space, but does not come into contact with the rocker arm 33 or 34, so that the rocker arm 35 strikes at the air (rocker arm non-engagement) . Accordingly, the rocker arms 33 and 34 make see-saw-like movements, following the rotation of the low-velocity cams 31a and 31b, respectively. As a result, the intake valves 11 and 12 are opened and closed with characteristics suitable for low-velocity operation of the engine (low-velocity operation mode).
On the other hand, when oil pressure in the cylinder 50 is increased by the oil pressure adjusting device, the piston 51 is lifted up, thereby entering an engaged state, and the opening 53 of the cylinder 50 is closed by the piston 51 [see FIG. 5(b)]. Thus, when the rocker arm 35 makes a see-saw-like movement, the engaging protrusion 35a provided at one end of the rocker arm 35 comes into contact with the side surface (engaging surface) 54 of the piston 51, and makes the rocker arms 33 and 34 develop a see-saw-like movement (rocker arm engagement). In this instance, the rocker arms 33 and 34 are driven by the rocker arm 35, while being separated from the low-velocity cam 31a and 31b, and move following the rotation of the cam shaft 31c, thereby opening and closing the intake valves 11 and 12 with characteristics suitable for high-velocity operation of the engine (high-velocity operation mode).
Here, in the above-described previous art, the piston 51 needs to have a comparatively large diameter, partly because a space large enough to make sure that the rocker arm 35 strikes the air is necessary during a low-velocity operation mode (at the time when the rocker arm is not engaged), and also partly because a space for arranging the return spring 52 which pushes down the piston 51 is necessary.
However, a large piston diameter increases the volume of oil necessary for switching operation modes (in particular, when switching from the high-velocity operation mode to the low-velocity operation mode), so that a longer time is required for switching the operation mode. Additionally, the engagement state between the piston 51 and the engaging protrusion 35a of the rocker arm 35 can be incomplete, so that the piston 51 is knocked out when it is being lifted due to a reactive force which drives a valve. As a result, the engaging protrusion 35a enters the opening, thereby switching the operation mode of the engine into the low-velocity operation mode.
Further, if the piston 51 is knocked out, the rocker arms 33 and 34 collides with the cam, thereby causing a knocking sound. If the impact is strong, the rollers 34a and 34b may be damaged.
With the foregoing problems in view, it is an object of the present invention to provide a valve mechanism for an internal combustion engine, with which valve mechanism it is possible to reliably perform switching between the low-velocity operation mode and the high-velocity operation mode without delay.